Surface acoustic wave elements in which inter digital transducer (hereinafter merely referred to as IDT) electrodes are formed on piezoelectric substrates have been conventionally known. The IDT electrode can mutually convert an electrical signal and a surface acoustic wave. In addition to the IDT electrode, the surface acoustic wave element can confine energy by arranging reflector electrodes at positions between which the IDT electrode is sandwiched and multiple-reflecting the surface acoustic wave excited by the IDT electrode using the reflector electrodes on both sides thereof.
In a duplexer used for a mobile terminal device such as a portable telephone, a surface acoustic wave device carrying a surface acoustic wave element having a high performance and capable of reducing size and weight has been used in recent years although a dielectric filter was conventionally used in principle. The input power level of the surface acoustic wave device has widened from a 10 mW level for an inter-stage filter in the mobile terminal device to 1 to 3 W levels required for the duplexer in the mobile terminal device with the spread of an application range of the surface acoustic wave device. In the surface acoustic wave device used in the duplexer for the mobile terminal device, therefore, the requirement of the input power level has increased.
On the other hand, in recent years, the operating frequency of the surface acoustic wave element has increased from several hundred megahertz to several gigahertzes. The line width of the IDT electrode included in the surface acoustic wave element has decreased with the increase in the operating frequency in inverse proportion to the frequency. Specifically, the line width of the IDT electrode is approximately 1 μm in an 800 MHz band, while being approximately 0.5 μm in a 1.9 GHz band. Therefore, an electrode finger formed in a comb shape in the IDT electrode requires fine processing with the increase in a frequency band.
The line width of the IDT electrode reduces, and the input power level of the surface acoustic wave element included in the duplexer increases, as described above, so that the power-resistant life of the surface acoustic wave element in the GHz band decreases by two or more figures than the surface acoustic wave device in the 800 MHz band.
In a case where the surface acoustic wave is excited and received using the fine electrode finger in the IDT electrode corresponding to the high frequency, when signal power applied to the surface acoustic wave device increases, a main surface of a piezoelectric substrate is distorted by the surface acoustic wave at the time of driving the surface acoustic wave device, so that an internal stress is created in the electrode finger in the IDT electrode. This internal stress causes a stress migration phenomenon to appear in the electrode finger. Therefore, the electrode finger is destroyed, and the IDT electrode is degraded.
As a result of stress migration, holes are accumulated in an Al grain boundary, so that a void and a hillock are produced. In the surface acoustic wave device, therefore, there arises characteristic degradation of performance such as propagation or resonance of the surface acoustic wave and destruction of the electrode finger.
Consequently, the surface acoustic wave element including the IDT electrode further requires durability against application of high power as applications are diversified. Therefore, an IDT electrode having an electrode finger whose durability is increased by laminating different materials has been developed in place of a conventional IDT electrode having an electrode finger formed of a single layer of a metal material such as Al or an Al alloy.
For example, in Japanese Unexamined Patent Publication No. 2001-217672, a configuration has been proposed in which an intermediate layer 112 formed of a material such as Ti (titanium) is arranged between an Al electrode layer 113 and a piezoelectric substrate 110, and is covered with a protective film (metal film) 114 for the purpose of restraining the occurrence of stress migrations in the IDT electrode 111, as shown in FIG. 13 in the present application.
Japanese Unexamined Patent Publication No. 2001-168671 discloses a structure of an IDT electrode formed by laminating a first metal layer and a second metal layer. A cross section of the IDT electrode is in such a trapezoidal shape that its lower surface closer to a piezoelectric substrate is wider than its upper surface.
Japanese Unexamined Patent Publication No. 3-217109 discloses a structure of a reflector electrode formed of a single metal layer and having a trapezoidal cross section.
International Publication No. WO2003/058813A1 discloses a structure of an IDT electrode formed by laminating a first metal layer having a trapezoidal cross section and a second metal layer having a rectangular cross section.
In the structure of the IDT electrode 111 in Japanese Unexamined Patent Publication No. 2001-217672, respective side surfaces of the intermediate layer 12 and the electrode layer 113 are formed almost perpendicular to the main surface 110a of the piezoelectric substrate 110. In this case, when the protective film 114 is formed by a film deposition method by which few evaporated species enter a masked area, such as an evaporation method, a material forming the protective film 114 is incident from a direction substantially perpendicular to the main surface 110a of the piezoelectric substrate 110, so that the protective film 114 is formed thin on a side surface of the electrode finger 111a. Therefore, generating of stress migrations cannot be controlled and as a result, fracture and stripping of the electrode may occur.
Although Japanese Unexamined Patent Publication No. 2001-168671 discloses the IDT electrode composed of two types of electrode layers formed of different materials and having a trapezoidal cross section, a shear stress is likely to arise on a boundary surface between the two types of electrode layers because the electrode layers differ in coefficients of thermal expansion.
Japanese Unexamined Patent Publication No. 3-217109 discloses only the structure of the reflector electrode having a trapezoidal cross section.
In International Publication No. WO2003/058813A1, when a shear stress arises on a boundary surface between two types of electrode layers because the electrode layers differ in coefficients of thermal expansion, each of the electrode layers cannot be prevented from being fractured and stripped, as in Japanese Unexamined Patent Publication No. 2001-168671.